Video camera systems are commonly employed for unattended recording of activities in a selected area. Some security applications, for example, have several video cameras connected to a recording system that periodically records a short period of video from each camera. Typically these cameras are designed to work in an auto-exposure mode, where a device called an electronic shutter varies the integration time of each video frame to match the ambient lighting conditions, or the camera is equipped with an auto-iris lens. In most cases, the cameras produce a continuous stream of standard video signals that contain two fields of video that are interlaced to create one frame of video. For example, the standard U.S. system, NTSC or RS-170 video, has 262.5 lines of video in one field interlaced with 262.5 lines in a second field, with the second set starting 1/60 sec. later than the first set. The video monitor or recorder used with these signals combines these fields alternately to produce a full frame of video for viewing. Other video systems exist that are called progressive scan systems. In these latter systems, a full frame of video is scanned without interlace, typically every 1/30sec.
In these systems, the video signals are represented by a voltage. In each field or frame, there will be a maximum voltage, representing the maximum amount of light the video system can convert into a signal, and a minimum voltage representing the minimum amount of light the system can reliably represent. In a scene viewed by the video camera, the image of the scene is converted to a video signal voltage representative of the scene with a range of voltages in between the maximum and minimum values. For a typical scene, and a particular value of auto-exposure, there may be portions of the scene that are too dark to contain any usable information, and other portions that are too light to be of use. In a surveillance application, for example, this can result in a situation where an essential part of the image is too light with a wash-out, or too dark, to be visible.
Heretofore one approach to overcoming the foregoing problem involves use of a combination dual exposure video camera. Combination dual exposure camera, systems comprise a camera that creates two different exposures of the same optical image via an electronic shutter and means for combining image data from both exposures into a single output field or frame. This results in a single image manufactured from the two different exposure parent images that spans a wider range of scene illumination values, but which is compressed within the voltage limits of a single video frame. An example of this type of camera system is disclosed by U.S. Pat. No. 5,420,635, issued May 30,1995 to M. Konishi et al. See also the following U.S. Patents for methods of creating high resolution images and/or compensating for variations in image luminance levels: U.S. Pat. No. 5,101,276, issued 31 Mar. 1992 to Y. Ohta et al.; U.S. Pat. No. 5,657,402 issued 12 Aug. 1997 to W. R. Bender et al.; and U.S. Pat. No. 5,920,657, issued 6 Jul. 1999 to W. R. Bender et al. A problem with the dual exposure image combining process is that although it expands the range of scene illumination values that are captured and utilized to manufacture the output fields or frames, it does so at the cost of reducing contrast from the resultant image by omitting various information contained in one or the other of the parent images. Also the combined image may be bleared if the object had movement. This omitted information is viewed in good faith as unimportant to the final image. However, such a judgment is not always proper or acceptable, particularly in video surveillance installations. Further, when such manufactured image fields are recorded, most of this dynamic range improvement is limited in post processing. This is because the information available in the manufactured image fields formed by combining signals no longer contains the full information present in the original pictures.
International Patent Application PCT/US00/40662, cited supra, discloses a video camera system and method whereby a stream of video signals is produced that defines a sequence of video fields or frames with video data representing at least two different exposure levels, with, for example, fields or frames of a first exposure level being interspersed among fields or frames of a second exposure level. Such video signals are utilized to generate a video display of the video data matching a particular set, or combination, of exposures. In addition to, or instead of, using the stream of video signals to provide a video display consisting of the optical images represented by selected video fields or frames, the system and method provides for recording the video signal output of the camera for subsequent use in generating a video display consisting of the optical image data represented by said selected video fields or frames. In other words, the display may be generated during or after recording. In comparison to the video signal output of a conventional dual exposure camera, the recorded stream of video signals of the system disclosed in International Patent Application PCT/US00/40662, comprising two or more pluralities of video fields or frames of different exposure levels, possesses a wider effective range of signal values due to the range of exposures recorded, covers a wider gamut of scene illumination, and has information of a higher dynamic range. By tailoring the number of different exposures to the camera and the video recorder, the full available dynamic range of the camera's video sensor may be preserved. Additionally, the system and method disclosed in International Patent Application PCT/US00/40662 offers the option of post-processing selected exposures to combine fields or frames of one exposure with fields or frames of other exposures, so as to achieve a result similar to that achieved with prior art methods for enhancing image resolution involving combining the video image data of two exposures into a single output field or frame.